The present application is based on Japanese priority application No.2002-086896 filed on Mar. 26, 2002, the entire contents of which are hereby incorporated by reference.
The present invention relates to fabrication method of Josephson devices that use a superconductor, and more particularly to the method of fabricating a Josephson device that uses an oxide superconductor while minimizing the variation of operational characteristics of the Josephson junction formed therein.
A superconductor has a unique property characterized by: 1) zero electric resistance; 2) complete diamagnetism; 3) Josephson effect, and application thereof is expected in various fields including electric power transfer, electric power generation, confinement of nuclear fusion plasma, magnetically levitated trains, magnetic shield, high-speed computers, and the like.
In 1986, Bednorz and Mueller discovered a copper oxide superconductor (La1-xBax)2CuO4 that has a very high superconducting transition temperature Tc of about 30K. Thereafter, numerous reports followed, reporting observation of superconducting transition occurring at high temperatures particularly in the systems of YBa2Cu3O7-y (Tc=90K), Bi2Sr2Ca2Cu3Oy (Tc=110K) Tl2Ba2Ca2Cu3Oy (Tc=125K), HgBa2Ca2Cu3Oy (Tc=135K), and the like. Currently, investigations are being made on the method of producing these materials as well as the properties and applications of these materials.
Particularly, the superconductor of YBa2Cu3Oy is thought as the most promising material in relation to the application to electron devices or conductor wires, in view of the preferable nature of the material such as absence of toxic elements such as Tl or Hg and small anisotropy of superconductivity.
In order to use Josephson effect in electron devices, on the other hand, it is necessary to establish the process of forming a Josephson junction by using a thin-film technology.
Generally, a Josephson junction constituting the essential part of a Josephson device is formed by a physical vapor deposition process in which a source material is dispersed into a gaseous phase by exciting the source material in a vacuum vessel, such as sputtering process; laser ablation process; vacuum evaporation deposition process; molecular beam epitaxy, and the like. Further, various structures are proposed for the Josephson devices that use a copper oxide superconductor, such as bicrystal structure, biepitaxial structure, step-edge structure, ramp-edge structure, laminated structure, and the like (S. Takada, Oyo-Buturi, 62, pp.443, 1993). Particularly, the Josephson device of the ramp-edge structure is thought most promising in view of the fact that a large driving power is achieved at the time of switching and that the critical current can be changed by controlling the thickness of the tunneling barrier layer (M. Hidaka, et al., Oyo-Buturi, 67, pp.1167, 1998).
An Ic.Rn product is an index representing the performance of a Josephson junction. Larger the Ic.Rn product, the better the operational speed of the Josephson junction. An Ic.Rn product is defined as a product of a critical current Ic and a resistivity Rn, wherein the critical current Ic is the maximum current possible in the superconducting state of a superconductor at a certain temperature, while the resistivity Rn is the resistivity for the case the superconducting state is lost and the superconductor has returned to a normal conducting state.
It should be noted that the Ic.Rn product is normalized by the size of the Josephson junction. Qualitatively, the Ic.Rn product represents the magnitude of the signal achieved at the time of the switching of the Josephson junction. By using the Josephson device of the ramp-edge structure, an Ic.Rn product larger than other device structures is obtained. Typically, YBa2Cu2Oy is used for the upper and lower superconducting electrodes. On the other hand, investigations are made also on the Josephson devices of the laminated structure, which is thought advantageous for formation of future large-scale integrated circuits. In both cases, the Josephson junction uses any of a PrBa2Cu3Oy layer, an Nb doped SrTiO3 layer or a damaged layer formed at the time of processing, for the junction barrier.
Recently, active investigation is being made on the certain type of Josephson devices that use a damaged layer formed at the time of processing, for the formation of the Josephson junction. This type of Josephson junction is called IEJ (Interface-Engineered, Junction). Reference should be made to B. H. Moeckly, et al., Appl. Phys. Lett.71, pp.2526, 1997). In the Josephson device having the ramp-edge structure, formation of a very thin layer having a thickness of 1-2 nm is confirmed by transmission electron microscopy. It is believed that this very thin layer functions as the Josephson junction, while detailed mechanism thereof is not yet understood (J. G. Wen et al. xe2x80x9cAdvances in Superconductivity XIIxe2x80x9d-Proc. ISS ""99, 10/17-19, pp. 984, 1999 in Morioka, Y. Soutome, et al., xe2x80x9cAdvances in Superconductivity XIIxe2x80x9d-Proc. ISS ""99, 10/17-19, pp.990, 1999, in Morioka).
In the foregoing IEJ Josephson devices, there can occur short-circuit in the Josephson junction at various locations in the case the process parameter at the time of formation of the Josephson junction is not appropriate. For example, the thickness of the damaged layer is so thin that control of thickness of the damaged layer is difficult.
It should be noted that the I-V characteristics of a Josephson device changes depending on the thickness of the junction layer therein. When the thickness of the Josephson junction is too large, a superconducting current cannot flow through the junction. When the thickness is appropriate, the superconducting current can flow through the junction by way of tunneling, without causing voltage difference across the junction, provided that the superconducting current is within the predetermined critical current Ic. When the magnitude of the current has exceeded this critical current Ic, there suddenly appears a voltage across the junction. In the state there is caused such a voltage, the I-V characteristic of the Josephson device approaches a straight line that crosses the origin. The I-V characteristics pertinent to such a Josephson device are called RSJ (Resistivity Shunted Junction) characteristics.
In the case there is a short-circuit in the Josephson junction due to the excessively small thickness of the barrier layer, on the other hand, there gradually appears a voltage across the junction when the current has exceeded the critical current Ic. In this case, the voltage is induced as a result of movement of the magnetic flux, and thus, the foregoing characteristics are called FF (Flux Flow) type I-V characteristics.
In order to realize a superconducting electron device that uses a Josephson device, it is necessary to produce a large number of Josephson junctions and devices having the foregoing RSJ characteristics and having a suitable critical current Ic. Further, the Josephson device is required to have a suitable Ic.Rn product. Particularly, the quantity Ic is sensitive to the junction structure or fabrication process, and thus, it is very important to establish the technology of suppressing the variation of the critical current Ic.
In order to achieve operation of a Josephson integrated circuit including therein 100 or more Josephson junctions, there is an estimate that the variance 1"sgr" of the junction characteristics has to be suppressed to 10% or less (J. Talvacchio, et al., IEEE Trans. Appl. Supercond.7, pp.2051, 1997) for the Josephson junctions included in the circuit.
Recently, there has been a report announcing the success of achieving the 1"sgr" value satisfying the foregoing requirement for an IEJ ramp-edge Josephson device.
More specifically, Satoh et al. achieved the variance 1"sgr" of 8% (1"sgr"=8%) over 100 Josephson junctions by using YBa2Cu3O7-y for the superconducting electrodes and (La0.3Sr0.7) (Al0.65Ta0.35)Oy for the insulating layer (T. Satoh, et al., IEEE Trans. Appl. Supercond. 9, 99.3141, 1999). The invention by Satoh et al. is disclosed in Japanese Laid-Open Patent Application 2000-150974.
According to the reference, the proposed process provides excellent junction characteristics because of the homogeneous barrier layer formation between the two superconducting electrodes within the thickness of 2 nm or less and because of incorporation of La into the barrier interface from the insulating layer at the time of etching process.
On the other hand, the amount of La thus incorporated is extremely small and it is not possible to confirm the existence of such La atoms even when a high-resolution analyzer transmission microscope, which conducts element analysis based on a characteristic X-ray emitted in response to irradiation of an electron beam having a beam diameter of as small as about 1 nm, is employed (J. G. Wen, et al., Appl. Phys. Lett., 75, pp.5470, 1999).
On the other hand, Soutome, et al., has successfully achieved the 1"sgr" value of 7.9% (1"sgr"=7.9%) over 100 Josephson junctions at 4.2K, by using YBa2Cu3O7-y for the superconducting electrodes and CeO2 for the insulation film (Soutome, et al., 62nd Annual Meeting of Japan Society of Applied Physics 14a-G-7, Sep. 11-14, 2001, Abstract No.1, pp.195). Soutome, et al. does not use a material containing La, and thus, the structure of Soutome, et al. achieves the foregoing result with the structure and process in which there occurs no La incorporation between the two superconducting electrodes.
In the industrial application of Josephson devices, it is mandatory to establish the technology of producing a number of Josephson junctions each having appropriate characteristics, with excellent reproducibility. Particularly, there is a demand of such a technology in the fabrication of future large-scale Josephson integrated circuits.
Conventionally, intensive efforts have been made to form Josephson junctions by using oxide high-temperature superconductors, which are characterized by a high Tc value, in the prospect of realizing superconducting electron devices operable at relatively high temperatures. However, the Josephson junctions obtained so far have suffered from the problem of large variation or scattering of operational characteristics, and because of this, integrated circuits of only a very limited scale have been tested so far. In order to construct an integrated circuit of larger scale, the variance 1"sgr" of about 8% is not sufficient, and there is an acute demand for the process capable of forming Josephson junctions with much reduced variation of operational characteristics.
Further, not much attention has been paid conventionally to the second superconducting layer, which is formed on the barrier layer of the Josephson junction.
Accordingly, it is a general object of the present invention to provide a method of forming a Josephson device wherein the foregoing problems are eliminated.
Another and more specific object of the present invention is to provide a fabrication process of a Josephson device capable of forming a stable and reliable Josephson junction therein with reduced variation of junction characteristics, as well as a Josephson device fabricated according to such a method.
Another object of the present invention is to provide a method of fabricating a Josephson device, comprising the steps of:
forming a first superconducting layer on a substrate;
forming an insulating film on said first superconducting layer; and
forming a second superconducting layer on said insulating film,
wherein said step of forming said second superconducting layer comprises the steps of:
removing said insulating layer at least from a predetermined area of said insulating layer so as to expose said first superconducting layer;
conducting, after said step of removing said insulating layer, a first step of forming said second superconducting layer; and
conducting a second step of forming said second superconducting layer on said second superconducting layer formed in said first step.
According to the present invention, reliable and stable Josephson devices are formed with reduced scattering of device characteristics, by conducting the first step, or initial phase, of formation of the second superconducting layer under the condition optimized for formation of a Josephson junction.
In the case of a Josephson device that uses a copper oxide superconductor, there has been a problem, associated with the characteristically small coherent length in such an oxide superconductor system, in that it is difficult to form a reliable barrier layer with reproducibility.
The inventor of the present invention has discovered, in the investigation that constitutes the foundation of the present invention, that the characteristics of the barrier layer in a Josephson junction is determined not only by the formation process of a non-superconducting layer constituting the barrier layer, but also by the deposition process of the second superconducting layer conducted thereafter. Particularly, the condition of film formation for the part of the second superconducting layer close to the barrier layer provides a profound effect on the characteristics of the Josephson junction.
Thus, in order to form a large number of Josephson junctions, and hence a large number of Josephson devices, on a substrate with uniform junction characteristics or device characteristics, it is not sufficient to achieve uniformity for the non-superconductor material forming the individual barrier layers. It is also necessary to achieve uniformity in the process of forming the superconducting layers provided on the barrier layers. Thereby, it should be noted that the best deposition condition for formation of a Josephson junction is not always equal to the optimum deposition condition of a high-quality superconductor.
In the present invention, the formation of the superconducting layer on the barrier layer is conducted in at least two steps. In the first step, the superconducting layer is formed under a condition optimized for maximum uniformity over the junctions on the substrate. After the first step, the deposition of the superconducting layer is continued with a condition optimized for maximum film quality.
Thereby, it is possible to conduct the step of removing the insulating film by removing a part of the first superconducting layer at the same time. By doing so, it is possible to use a modified layer formed on the very surface part of the first superconducting layer as a result of the removing step of the insulating film for the barrier layer of the Josephson junction. With this procedure, it is possible to form extremely uniform barrier layers in the Josephson junctions formed over a wide area of the substrate, and the variation of characteristics of the Josephson junctions, and hence the variation of characteristics of the Josephson devices, is successfully minimized.
Further, it is also possible to conduct the first step of forming the second superconducting layer such that the difference of condition of film formation over different locations on the substrate is minimized as compared with the case of conducting the second step of forming the second superconducting layer.
Thereby, it should be noted that the first step of forming the second superconducting layer can be conducted by using a physical vapor deposition process that uses a plurality of evaporation sources.
Further, it is possible to conduct the first step of forming the second superconducting layer by way of a physical-vapor deposition process conducted such that there is formed an obstacle between an evaporation source and the substrate.
Further, it is possible to conduct the first and second steps of forming the second superconducting layer by way of a physical vapor deposition process in such a manner that the distance between the substrate and the evaporation source is increased in the first step as compared with the second step.
Alternatively, it is possible to conduct the first and second steps of forming the second superconducting layer by way of a physical vapor deposition processes in such a manner that the excitation energy of the evaporation source is reduced in the first step as compared with the second step.
Alternatively, it is possible to conduct the first and second steps of forming the second superconducting layer such that an ambient pressure is increased in the first step as compared with the second step.
Further, it is possible to conduct the first step of forming the second superconducting layer by any of a sputtering process, a vacuum evaporation deposition process and a molecular beam epitaxy process and conduct the second step of forming the second superconducting layer by a laser ablation process.
In a physical vapor deposition process in which a source material is dispersed into a gaseous phase by causing excitation of the source material in a vacuum vessel, it is ideal to cause the source material particles, which are emitted uniformly from an evaporation source provided on a surface, to fall on a substrate disposed parallel to the foregoing surface when it is desired to cause deposition of the source material particles with exactly the same condition over any area of the substrate. In practice, however, it is difficult to construct such a facility. In many cases, deviation of deposition condition is more or less unavoidable. This, however, does not mean that improvement for reducing the deviation of deposition condition is not possible.
In order to homogenize the energy of the particles that fall on a substrate in a physical vapor deposition process, there are generally two alternatives of: 1) provide plural excitation regions or plural high-density regions of high-energy particles; and 2) provide a sufficient distance between the excitation region of the source material or the high-density region of the high energy particles and the substrate.
In the case of laser ablation process, the foregoing excitation region of the source material corresponds to the region of the target to which the laser beam is directed while the high-density region of the high-energy particles corresponds to the plume, which appears at the time of the laser beam irradiation.
In the case of the sputtering process, the excitation region corresponds to the part of the target hit with plasma (called erosion), while the high-density region of the high-energy particles corresponds to the plasma. In the case of vacuum evaporation deposition process or molecular beam epitaxy, the excitation region corresponds to the evaporation source and the high-density region corresponds to the region near the evaporation source.
Further, it is effective also to change the film formation process between the first step and the second step. Generally, the uniformity of energy of the particles depositing on a substrate is increased with the order of: laser ablation process; sputtering process; vacuum evaporation deposition process or molecular beam epitaxy process. Thus, it is possible to conduct the initial deposition process by employing the process of high uniformity, followed by the substantial deposition process by using a process of lower uniformity.
Thus, according to the present invention, it is possible to suppress the variation of device characteristics of the Josephson devices, particularly the variation of critical current Ic, over a wide area of the substrate by conducting the first step of the formation process of the second superconducting layer under the condition chosen such that a uniform energy distribution is achieved for the deposited molecules or particles over wide area of the substrate. Thereby, reliable Josephson devices can be obtained with uniform device characteristics.
The Josephson devices thus fabricated are characterized by the variance 1"sgr" of the critical current Ic of 8% or less. Thus, according to the present invention, it becomes possible to suppress the value of variance 1"sgr" of the critical current Ic of the Josephson junctions to 8% or less over 100 Josephson junctions, by conducting the first step of the process of forming the second superconducting layer under the condition set such that a uniform energy distribution is realized for the deposited molecules or particles over a wide area of the substrate. Thus, the present invention successfully realized stable and reliable Josephson devices having a Josephson junction therein.
Other objects and further features of the present invention will become apparent from the following detailed description when read in conjunction with the attached drawings.